Metal-hydrogen Interactions Research Articles

Structural study of metal–hydrogen interactions in cubic PrH 2+ x and rare-earth analogues

Neutron diffraction data on cubic PrD 2.92 show that the deuterium atoms in the octahedral interstices are displaced by 0.31 Å from the centre towards the faces. This leads to a considerable shortening of the Pr–D bonds (2.57 Å as compared to 2.74 Å for the centre position) and thus to a gain of energy. A comparison with other cubic RD 2+ x structures (R=La, Ce, Nd) shows that the displacements and the shortening of the R–D bonds decrease as the atomic size of R decreases, whereas the shortest contact distances between the deuterium atoms in octahedral and tetrahedral interstices remain nearly constant (∼2.1 Å). These findings suggest that as one goes from light to heavy R elements (or from low to high hydrogen contents) the energy gain due to R–H bond shortening (or the addition of new R–H bonds) is increasingly offset by the energy loss due to repulsive H–H interactions. This trend is consistent with the decreased homogeneity range of the cubic RH 2+ x phase and the appearance of the trigonal RH 3 phase in R–H systems containing heavy R elements.

Theoretical modeling of metal–hydrogen interactions in Pd clusters

There is an enhancement in hydrogen solubility and narrowing of the two-phase plateau pressure region in the pressure-composition isotherm of Pd clusters compared to bulk Pd. Experimental data on the hydrogen solubility in bulk Pd and Pd clusters at 310 K have been utilised to obtain thermodynamic properties. The relative chemical potential of dissolved hydrogen (H) in the metal at infinite dilution and the apparent H–H interaction energy is lower in the case of Pd clusters. The solubility of H in Pd clusters was analysed by Fermi–Dirac statistics, using both single-site and two-site models. The two-site model assumed that H occupies both subsurface and interior sites. The subsurface sites have a lower energy compared to the interior sites thereby indicating that H would preferentially occupy these sites. The higher solubility of H in Pd clusters is due to the availability of subsurface sites of lower energy compared to bulk sites.

Chemical bond state and hydride stability of hydrogen storage alloys

Abstract The electronic structures for typical hydrogen storage alloys, LaNi 5 , TiFe, ZrMn 2 , Mg 2 Ni and b.c.c.V, all containing a variety of alloying elements, M, are investigated by the DV-Xα molecular orbital method. It is shown that atomic interactions of controlling the stability of their hydrides, depend strongly on the way how crystal structural evolution takes place in the course of hydrogenation. For example, when the hydride structure is the derivative one of the starting alloy, the metal–metal interaction and its change with hydrogenation will determine the stability of the hydride. LaNi 5 , TiFe and ZrMn 2 belong to this group. On the other hand, if the crystal structure of the hydride is completely different from that of the starting alloy, the importance of the metal–hydrogen interaction increases, and in case of b.c.c.V, only the metal–hydrogen interaction is responsible mainly for the relative stability of alloyed VH 2 . In case of Mg 2 Ni, both the metal–metal and the metal–hydrogen interactions control the stability of the hydride, Mg 2 NiH 4 .

Monte Carlo simulation of hydrogen diffusion in C15 Laves phase intermetallic compounds

Abstract We present preliminary results from Monte Carlo simulations of hydrogen diffusion in C15 Laves phase compounds. The incoherent quasi-elastic neutron scattering function, Sinc(Q, ω), is evaluated for the case of non-interacting particles diffusing on g-sites and is compared to the analytical model proposed by Rowe et al. [J.M. Rowe, K. Skold, H.E. Flotow, J. Phys. Chem. Solids 32 (1971) 41–54]. A good level of agreement, at low Q, is obtained when the tracer correlation factor is accounted for. A value of 0.00238 A2 per Monte Carlo cycle is obtained for the tracer diffusion coefficient. A brief discussion of the extension of this simple model to include metal hydrogen interactions is also given.

Characterization of trapped hydrogen in exfoliation corroded aluminium alloy 2024

Department of Mechanical and Industrial Engineering, University of Thessaly, Pedion Areos,GR-38334 Volos, Greece(Received September 21, 1998)(Accepted in revised form September 9, 1999)Keywords: Aluminium; Hydrogen; Trapping statesIntroductionStress corrosion and corrosion fatigue represent major threats to the structural integrity of aircraftstructures and have been extensively investigated [1–3]. Among others, Pantelakis et al. [4] observeddramatic degradation of toughness and ductility of Al-Li alloys 2091 and 8090, as well as conventional2024 alloy in several types of accelerated corrosion tests. Based on a comparison of mechanicalproperties (in particular fracture energy density and fracture strain) of naturally and artificially corrodedspecimens, they chose the exfoliation accelerated corrosion test to simulate natural corrosion [5].The above behavior has been attributed by many investigators to hydrogen embrittlement. Hydrogen-produced during corrosion- has been accused of diffusing to the interior, leading to material embrittle-ment through hydrogen-metal interactions [6,7,8]. Despite the lack of a universally accepted hydrogenembrittlement mechanism, a generally recognized common feature is that some critical concentration ofhydrogen must build-up at potential crack sites, for failure to initiate. Thus, the distribution of hydrogeninside the metal and its pattern of migration are of paramount importance in understanding thephenomena and designing alloys with improved behavior.It has been shown [9,10] that lattice defects (vacancies, dislocations, grain boundaries) and precip-itates provide a variety of trapping sites for diffusing hydrogen. Hydrogen traps have mechanisticallybeen classified by Presouyre [11] as reversible and irreversible, depending on the steepness of theenergy barrier needed to be overcome by hydrogen to escape from the trap. For example, during adegassing experiment reversible traps will release hydrogen continuously, while irreversible ones willdo so only after a critical temperature has been reached. This is the temperature at which the probabilityof a single jump out of the steep trap becomes nonnegligible.Reversible and irreversible traps may play different roles during an actual experiment [12]. Inparticular, irreversible traps will always act as sinks for hydrogen, whereas reversible traps may act assinks or sources depending on initial hydrogen charging of the lattice. A uniform distribution ofirreversible traps is believed to provide a beneficial effect in alloy behavior under embrittlingconditions, by arresting diffusing hydrogen and thus delaying its build-up at the crack sites [13].

Effect of surface oxidation on the solution of hydrogen in vanadium

The formation of vanadium surface oxides and their influence on the absorption of hydrogen were studied by XPS and TDMS methods. Vanadium hereby serves as a model system for hydrogen metal interaction. Different stable and unstable oxides indicated by the oxidation number of vanadium have been investigated: oxidation number +5 corresponding to stable V 2 O 5 ; oxidation number +3 corresponding to stable V 2 O 3 and lower than +3, corresponding to unstable oxides. Exposure of a cleaned V sample to different oxygen dosages (1 L - 1000 L, P O2 = 1 × 10 -4 Pa) at room temperature leads to the formation of unstable oxides with oxidation numbers smaller than 3. V-O bondings break up at temperatures higher than 290°C and oxygen desorbs. Besides the dosage the oxide formation is also influenced by the temperature. All these oxides act as surface barriers and prevent the absorption of hydrogen by vanadium.

Design of hydrogen storage alloys with the aid of molecular orbital method

The electronic structures of hydrogen storage alloys are calculated by the DV-Xα molecular orbital method. The results revealed that hydrogen interacts more strongly with hydride-non-forming elements, B, (e.g. Ni, Mn, Fe) than hydride-forming elements, A, (e.g. La, Zr, Ti, Mg) in every hydrogen storage alloy, despite there being a larger affinity of A elements for hydrogen than B elements in the binary metal-hydrogen system. The presence of such a metal-hydrogen interaction is a characteristic of hydrogen storage alloys. Furthermore, the A/B compositional ratio of hydrogen storage alloys can be understood in terms of a simple parameter, 2Bo(A-B)/[Bo(A-A)+Bo(B-B)], where the Bo(A-B), Bo(A-A) and Bo(B-B) are the bond orders between atoms given in the parentheses.

Part I-the effects of pre-dissolved hydrogen on cleavage and grain boundary fracture initiation in metastable beta Ti-3Al-8V-6Cr-4Mo-4Zr

The effects of electrochemically pre-dissolved hydrogen on room-temperature fracture initiation in Beta-C titanium (Ti-3Al-8V-6Cr-4Mo-4Zr wt pct) have been investigated using circumferentially notched tensile specimens. Finite element-based analysis of notch stress fields was used to define relationships between the local threshold stress for crack initiation vs total internal hydrogen concentration. The as-received, solution heat treated (ST, σy.2 pct=865 MPa) and the ST + peak-aged conditions (STA, σy.2% pct=1260 MPa) were compared after defining the relationships between the fracture process zone hydrogen concentration, hydrogen-metal interactions (i.e., hydrostatic stress field occlusion, trapping, hydriding), and the resulting fracture initiation behavior of each. Solutionized + peak-aged (β+α) Beta-C fractured intergranularly above total hydrogen concentrations of ∼1000 wt ppm. (5.1 at. pct). A fracture mode consistent with cleavage occurred at ∼2100 wt ppm. (10.7 at. pct). Solutionized Beta-C resisted hydrogen-assisted cracking (e.g., did not crack intergranularly) but was not immune; cleavage cracking was provoked at ∼4000 wt ppm. (20.4 at. pct). Coldworked ST Beta-C (CW, σy.2 pct=1107 MPa) did not crack intergranularly; fracture initiation behavior was similar to the ST condition regardless of specimen orientation. This suggests that high yield strength alone does not account for the susceptibility to intergranular cracking observed in the STA β+α condition. Stroke-rate studies and X-ray diffraction investigation of H partitioning suggests that equilibrium hydriding and/or irreversible trapping does not singularly control intergranular fracture initiation of the STA condition. Fractographic evidence and finite element results show that a finite plastic zone exists prior to intergranular fracture of the STA condition. This suggests that a criterion for fracture that incorporates plastic strain and stress should be considered.

Ab initio study of iron and iron hydride: III. Vibrational states of H isotopes in Fe, Cr and Ni

Adiabatic potentials, energy levels and wavefunctions for collective vibrational states of hydrogen isotopes in monohydrides of transition metals with face-centred cubic lattices, and NiH, and with body-centred cubic lattices, and CrH, are investigated by means of ab initio total-energy calculations in the local density and local spin-density approximations (LDA, LSDA). The study for the different transition-metal monohydrides, including PdH and NbH studied earlier, yields a general insight into the microscopic vibrational potential wells: the topology of their spatial shapes is specific to the metal lattice, but their depths and curvatures change quantitatively in a systematic manner through the transition-metal series. The calculated excitation energies agree very well with results of inelastic neutron scattering (INS) experiments both for non-magnetic NiH, treated within the LDA, and ferromagnetic , treated within the LSDA. The theoretical data for the two considered hydrides, with bcc structure, for which corresponding experimental data do not exist, provide an ab initio database for the construction of metal-hydrogen interaction models, e.g., for studies of self-trapped vibrational states of isolated H atoms in transition metals.

Solubility isotherms of hydrogen in epitaxial Nb(110) films

The solubility isotherms of hydrogen in epitaxial Nb(110) films have been determined via in situ x-ray lattice-parameter measurements. The Nb films were grown by molecular-beam epitaxial techniques on ${\mathrm{Al}}_{2}$${\mathrm{O}}_{3}$(112\ifmmode\bar\else\textasciimacron\fi{}0) substrates with thicknesses ranging from 32 to 527 nm. All isotherms can be well described by the standard expression for the solubility of a monatomic lattice gas. Aside from the metal-hydrogen interaction, which appears to be bulklike, all other properties exhibit a strong thickness dependence, including the hydrogen-hydrogen interaction, the critical temperature for the \ensuremath{\alpha}-${\mathrm{\ensuremath{\alpha}}}^{\ensuremath{'}}$ transition, and the maximum lattice-parameter change that can be reached in saturation. For the critical temperature we find a scaling with the film thickness, which most likely is due to the elastic boundary condition provided by the rigid substrate. The clamping of the epitaxial film to the substrate hinders critical fluctuations of macroscopic hydrogen density modes from developing, thereby lowering the critical temperature for the \ensuremath{\alpha}-${\mathrm{\ensuremath{\alpha}}}^{\ensuremath{'}}$ transition. \textcopyright{} 1996 The American Physical Society.

Effects of cathodic overprotection on some mechanical properties of a dual-phase low-alloy steel in sea water

The excessive protection of a dual-phase low-alloy steel against corrosion attack in sea water by means of the impressed — current cathodic protection system was studied with a view to determining the effects of overprotection on some mechanical properties of the material. Although a potential of −825 mV with respect to a saturated calomel electrode (mV sce) is required for protection, the applied potentials were varied to reach −1400 mV sce. The results of the study have shown that the hydrogen-metal interaction, effected by hydrogen entry into the interstices within the metal crystal lattice in the course of the application of the overprotection potentials may significantly influence the strength behaviour of the steel by virtue of the influence of the presence of hydrogen on the rate of increase of lattice dislocation multiplication. With the exception of ductility, overprotection at potentials not exceeding −1100 mV sce may not result in an overall decline in the mechanical properties. Ductility could be retained if operating at potentials not beyond −900 mV sce.

The interaction of hydrogen with a cobalt(101̄0) surface

The adsorption of hydrogen on a cobalt(101̄0) surface was investigated in ultrahigh vacuum (UHV) between 85 and 500 K using Video-LEED, temperature-programmed thermal desorption (TPD), work function (ΔΦ) measurements, and high-resolution electron energy loss spectroscopy (HREELS). Between 90 and 200 K, hydrogen adsorbs dissociatively with high sticking coefficient (s0≥0.8) via precursor kinetics and forms, with increasing exposure, a c(2×4), a p2mg (2×1) and a (1×2) LEED structure (hydrogen coverages ΘH=0.5, 1.0, and 1.5, respectively). While the first two structures represent true ordered hydrogen phases there is strong evidence that the (1×2) phase is reconstructed, likely in a paired-row configuration. The formation of the (1×2) phase is slightly thermally activated; its decomposition produces a sharp thermal desorption maximum (α state) appearing on the low-energy side of a β-TPD signal which reflects the hydrogen desorbing from the unreconstructed surface. The activation energies for desorption from the α and β states are 62 and 80 kJ/mol, respectively. Chemisorption in the β state [(2×1) phase up to ΘH=1.0] is associated with a ΔΦ of +207 meV, while the fully developed (1×2) reconstructed phase (α state) causes a ΔΦ of approximately −122 meV resulting in an overall work function change of +85 meV at saturation. From HREELS, we determine the H adsorption site in all superstructures to be threefold with a local CS symmetry. Our results are discussed and compared with previous findings for similar metal–hydrogen interaction systems.

Investigation of thermodynamic properties of the TiH system using molten salt electrolytes containing hydride ions

A novel molten salt technique using alkali halide electrolytes, which contain hydride ions, was used to study thermodynamic properties of the TiH system at intermediate temperatures (300–415 °C). This electrochemical technique offers a simple, fast, precise and sensitive method for investigation of metal-hydrogen interactions with minimal surface interference at temperatures where no suitable electrolyte was available previously. We report results that extend to a lower hydrogen activity range than any previous studies. The results agree with those measured by others using conventional gas-volumetric or calorimetric techniques.

Vibrational spectra and thermal decomposition of methylamine and ethylamine on nickel(111)

The bonding and geometry of methylamine (CH{sub 3}NH{sub 2}) and ethylamine (CH{sub 3}CH{sub 2}NH{sub 2}) on Ni(111) have been investigated with high-resolution electron energy loss vibrational spectroscopy (HREELS). Both amines adsorb molecularly at 150 K through the nitrogen lone pair. Significant metal-hydrogen interactions in the alkyl chain were indicated by {open_quotes}softened{close_quotes} C-H stretching modes with frequencies shifted to 2660-2680 cm{sup {minus}1}. Temperature-programmed desorption (TPD) and HREELS were used to monitor their desorption and thermal decomposition on the Ni(111) surface. Both CH{sub 3}NH{sub 2} and CH{sub 3}CH{sub 2}NH{sub 2} are dehydrogenated in the temperature range 300-400 K. CH{sub 3}NH{sub 2} is dehydrogenated to HCN at about 330 K, which further decomposes above 360 K. CH{sub 3}CH{sub 2}NH{sub 2} is dehydrogenated to CH{sub 3}CN, initially by {alpha}-C-H bond scission, leading to desorption of that molecule at 350 K. On the basis of their spectra, the authors propose a mechanism for the dehydrogenation processes of CH{sub 3}NH{sub 2} and CH{sub 3}CH{sub 2}NH{sub 2} on Ni (111). 33 refs., 13 figs., 3 tabs.

The activation and elimination of H2 by Zr complexes

An ab initio analysis of the reaction of molecular hydrogen with the Zr-imido complex (NH2)2Zr NH is reported. Several interesting points are noted. The calculated stretching frequency of the ZrN bond in (NH2)2ZrNH is 860 cm−1 when properly scaled to account for electron correlation effects. The value supports the assignment of an infrared (IR) band at 865 cm−1 to the ZrN stretch of a tetrahydrofuran (THF) adduct of the putative reactive intermediate (NHSi′)2ZrNSi′. Although a weakly bound, H2 complex is found, the interaction is small (1.3 kcal mol−1) compared with experimentally characterized H2 complexes. Variations in bond lengths and intrinsic stretching frequencies demonstrate that π-bonding for amido (NH2) ligands can be substantial in a coordinatively saturated complex. H2 activation by the bis(amido)imido reactive intermediate is calculated to be significantly more favorable by the 1,2-addition of H2 across the ZrN bond to form the tris(amido)hydride than a sigma-bond metathesis pathway. The transition state (TS) for the addition of H2 across the ZrN bond of the bis(amido )imido complex is 9.8 kcal mol−1 above the charge transfer (CT) complex, (NH2)2ZrNH  H2 at the MP2 level; the reverse process, extrusion of H2 from the tris(amido)hydride has a 28.2 kcal mol−1 barrier. The geometry of the four-center TS is of interest, deviating markedly from that of a square and being more “kite” shaped (i.e., one obtuse and three acute angles). Mulliken Bond Overlap Populations suggest that there is some interaction between the Zr and the H atom being transferred (Hl) in the various TSs studied. It is suggested that the interaction plays an important role in the ability of the Zrimido complexes, and indeed other high-valent, multiply bonded complexes, to activate XH (XH, C, Si, N, and H) bonds. The design of materials and catalysis precursors which enhance the metalhydrogen interaction in the TS could conceivably lead to lower chemical vapor deposition (CVD) processing temperatures and higher catalytic activities. © 1992 John Wiley & Sons, Inc.

Energetics and electronic structure of hydrogenated clusters

Small atomic clusters constitute a new class of matter whose atomic structure and electronic properties depend strongly on their size and composition. The interaction of these clusters with hydrogen is dominated by their surface-like character and can deviate significantly from the corresponding bulk metal-hydrogen interaction. Some of the salient features of clusters and their interaction with hydrogen are reviewed. Analogy is also drawn between metal hydrides vs. cluster hydrides.

The quantum simulation of metal-hydrogen systems and the analysis of diffraction experiments

Abstract The path-integral simulation technique is used to study an empirical model for hydrogen and deuterium in niobium. This simulation method fully incorporates quantum effects, which are known to play an important role in the behaviour of hydrogen in metals, especially at low temperatures. It is argued that the density distribution of hydrogen is a key quantity in understanding metal-hydrogen systems since it depends sensitively on the metal–hydrogen interaction and since it is directly related to structure factors measured by diffraction experiments. We present simulation results for the density distribution of the niobium–hydrogen and niobium–deuterium systems in real space at 300,423, 500 and 700 K. We calculate the relevant structure factors and compare with existing diffraction data. We conclude firstly that the simulation results can be directly compared against diffraction data and such comparisons provide an important test of the interaction model on which the simulation is based and secondly...

Effective-medium calculations for hydrogen in Ni, Pd, and Pt

The effective-medium theory is applied to a study of the energetics of the hydrides of Ni, Pd, and Pt, stressing the properties of ${\mathrm{PdH}}_{\mathrm{\ensuremath{\theta}}}$ for 0\ensuremath{\le}\ensuremath{\theta}\ensuremath{\le}1. The calculated heat of solution and the heat of hydride formation for the three systems agree very well with experiment. We determine the favored structure for ${\mathrm{PdH}}_{\mathrm{\ensuremath{\theta}}}$ by calculating the total energy and lattice expansion of different configurations. Vibrational frequencies and diffusion barriers of H in Pd are also treated. A simple and transparent physical picture of the hydrogen-metal interaction is developed. From the calculated energetics we make a model calculation of the phase diagram of hydrogen in palladium in qualitative agreement with experiment. On this basis we propose a new explanation of the peculiarities of the Pd-H system.

Differentiation of two stages during establishment of strong metal-support interaction in rhodium/titania catalysts

It is known that treatment under H{sub 2} at 773 K of catalysts consisting of group VIII metals supported on TiO{sub 2} gives rise to strong metal-support interaction (SMSI behavior), which inhibits hydrogen adsorption on the metal. Its origin has been related to the formation of metal-titania bonding and/or to the coverage of metal by TiO{sub x} species; however, the nature of this phenomenon is uncertain, and little information about the influence of the reduction degree of titania is yet available. In this work, the analysis of the intensity of both lines as a function of reduction temperature has permitted evaluation of the amount of proton species on the support and the capacity of the metal to chemisorb hydrogen. In addition, chemical shift of line B has been used to follow changes in the hydrogen-metal interaction, which evidence modifications in electronic properties of the metal during SMSI establishment.

A new ab initio potential energy surface for hydrogen atom on ruthenium(0001) and its use for variational transition state theory and semiclassical tunneling calculations of the surface diffusion of hydrogen and deuterium

We have applied variational transition state theory with a semiclassical tunneling method to calculate the rates of hydrogen and deuterium diffusing on a Ru(0001) surface. The hydrogen-metal interaction is modeled by a pairwise potential which has been fitted to electronic structure calculations based on a local density approximation and a pseudopotential. We present diffusion coefficients and kinetic isotope effects fort the temperature range from 80 to 800 K using the fitted potential and also for two modified potentials ― one in which the potential along the minimum-energy path is scaled by a constant and one in which the pairwise potential parameters are modified to change the barrier height. The results for all three potentials are compared to experimental rate coefficients determined recently by laser-induced thermal desorption